The present invention relates to an adhesive tape free of generation of dioxin or a toxic gas upon incineration, which is superior in flexibility, resistance to thermal deformation, flame resistance and resistance to surface whitening, and which has suitable stretchability and mechanical strength, and to a substrate for the adhesive tape.
A conventional adhesive tape comprising a substrate for an adhesive tape (hereinafter to be also referred to simply as a substrate), which is made from polyvinyl chloride (hereinafter to be abbreviated as PVC), is superior in mechanical properties (particularly flexibility and stretchability), flame resistance, resistance to thermal deformation, electrical insulation property, forming processability and the like. Combined with its beneficial economical aspect, the above-mentioned tape has been widely used as an insulating tape for electric equipment used in the fields of vehicles, such as automobile, train, bus and the like, aircraft, ship, house, plant and the like. Particularly, an adhesive tape to be wound around a wire harness to be used for electric wires in automobile and the like, a coil for household electric appliances, an electric wire and the like, is required to show high flame resistance (oxygen index of not less than 25%), high resistance to thermal deformation (heat deformation rate of 65% or less at 100xc2x0 C., 4.90N load), and, from the aspect of workability during tape winding around an object to be adhered to, suitable stretchability and suitable mechanical strength, that enable winding of a tape while stretching it with ease without breakage of the tape, and, after winding, cutting of the tape easily with hand (hereinafter to be also referred to as hand cutting property). To meet these requirements, an adhesive tape using PVC as a substrate has been widely used.
Given the recent rise of consciousness toward environmental problems, however, there is a movement to reduce use of PVC and replace it with a material causing less environmental burden. This is because PVC is suspected of generating dioxin and toxic gas, such as chlorine gas, upon incineration. To replace PVC, the use of a polyolefin resin as a substrate has been considered, because it less likely generates dioxin or a toxic gas, such as chlorine gas, upon incineration. However, polyolefin resins easily burn as compared to PVC and require addition of a flame retardant. In general, inorganic metal compounds such as metal hydroxides causing less environmental burden (e.g., magnesium hydroxide, aluminum hydroxide etc.) and the like are added as inorganic flame retarders.
When a substrate is obtained by adding an inorganic flame retardant to a polyolefin resin, polypropylene, ethylene-propylene copolymer, high density polyethylene and the like, having a relatively high melting point, are not suitable as the polyolefin resin to which the inorganic flame retardant is to be added, in consideration of the flexibility and stretchability necessary for an adhesive tape. Instead, a polyolefin resin having a low melting point is preferably used, such as low density polyethylene, linear low density polyethylene, ultra-low density polyethylene, middle density polyethylene, ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer and the like. However, because these low melting point polyolefin resins are easily deformed by heat, and because they stretch too much, even if they are certainly flexible, an adhesive tape using a low melting point polyolefin resin as a substrate shows poor resistance to thermal deformation and hand cutting property.
As a method for improving resistance to thermal deformation of a resin film, there have been known a method comprising exposing the film to ionizing radiation, a method comprising previous addition of a crosslinking agent, such as organic peroxide (e.g., dicumylperoxide), to a resin and vapor heating of a film after forming to provide a crosslinking structure in the film, and the like. These methods also increase the number of production steps and production cost to the level that makes their practical use unfeasible.
A different method comprises the use of a mechanically blended polymer mixture of polypropylene, ethylene-propylene copolymer, high density polyethylene and the like, having a relatively high melting point, and an elastomer consisting of ethylene copolymer, such as EPM (ethylene-propylene rubber), EBR (ethylene-butene rubber), EPDM (ethylene-propylene-diene copolymer rubber) and the like. When a flame retardant is added to this polymer mixture, a molded product thereof becomes very stiff and lacks flexibility at room temperature, making stretchability strikingly low and resistance to thermal deformation insufficient.
For sufficient flame resistance to be achieved, an inorganic flame retarder needs to be added in a comparatively large amount to the polyolefin resin. The addition of the comparatively large amount of the inorganic flame retarder results in poor stretchability of the substrate and reduced mechanical strength (particularly strength at break). The addition of the inorganic flame retarder also causes whitening of the surface of a substrate when it is brought into contact with an object or when it is stretched. The whitening of the surface of the substrate not only impairs the appearance of the product obtained by winding an adhesive tape but also degrades the insulating property and mechanical property of the adhesive tape. Therefore, there is a strong demand for an improvement.
When an inorganic metal compound surface-treated with oleic acid, stearic acid or sodium salt etc. of these is used as a flame retarder, the inorganic metal compound has an improved dispersibility in a polyolefin resin, thereby reducing the amount of the inorganic metal compound necessary for producing sufficient flame resistance, as a result of which the mechanical property of the substrate can be improved. This attempt can improve the stretchability of the substrate somewhat but is ineffective for the prevention of the degradation of the mechanical strength (strength at break) of the substrate and whitening of the surface of the substrate.
It is therefore an object of the present invention to provide an adhesive tape free of generation of dioxin or a toxic gas upon incineration, which is superior in flexibility, resistance to thermal deformation, flame resistance and resistance to surface whitening, and which has suitable stretchability and mechanical strength, as well as a substrate to be used for this adhesive tape.
According to the present invention, (i) a thermoplastic resin having a carbonyl oxygen atom in the molecular skeleton, (ii) a polymer alloy containing an ethylene component and a propylene component, and (iii) an inorganic metal compound surface-treated with a silane coupling agent in combination, are used as components to constitute the substrate of an adhesive tape, and the substrate is made to be substantially free of halogen atom. As a result, a substrate for an adhesive tape can be obtained, which is free of generation of dioxin or a toxic gas upon incineration and.whitening of the surface upon contact with other objects and upon stretching, and which is superior in flexibility, resistance to thermal deformation and flame resistance, and has suitable stretchability and mechanical strength that afford fine winding workability and hand cutting property of the adhesive tape.
Accordingly, the present invention provides an adhesive tape comprising a substrate and an adhesive layer formed on at least one side of the substrate, wherein the substrate comprises the following Component A, Component B and Component C but substantially no halogen atom:
Component A: a thermoplastic resin having a carbonyl oxygen atom in the molecular skeleton
Component B: a polymer alloy containing an ethylene component and a propylene component
Component C: an inorganic metal compound surface-treated with a silane coupling agent.
The preferable embodiments are as follows.
The Component B has a dynamic storage modulus (Exe2x80x2) at 80xc2x0 C. of not less than 40 MPa and less than 180 MPa, and a dynamic storage modulus (Exe2x80x2) at 120xc2x0 C. of not less than 12 MPa and less than 70 MPa.
The Component B has a dynamic storage modulus (Exe2x80x2) at 23xc2x0 C. of not less than 200 MPa and less than 400 MPa.
The Component A is an ethylene copolymer or a metal salt thereof, having a melting point of not more than 120xc2x0 C., which is obtained by polymerizing a vinyl ester compound, or an xcex1,xcex2-unsaturated carboxylic acid or a derivative thereof, or the vinyl ester compound and the xcex1,xcex2-unsaturated carboxylic acid or a derivative thereof.
The Component C is an inorganic metal compound surface-treated with 0.1-5.0 wt % silane coupling agent per the weight of the inorganic metal compound.
The Component A and the Component B are mixed at a weight ratio (A:B) of 1:9-8:2, and the Component C is added in an amount of 80-200 parts by weight per 100 parts by weight of the total weight of the Component A and the Component B.
The adhesive tape has a dynamic storage modulus (Exe2x80x2) at 80xc2x0 C. of not less than 25 MPa and a dynamic storage modulus (Exe2x80x2) at 120xc2x0 C. of not less than 10 MPa.
The substrate has a dynamic storage modulus (Exe2x80x2) at 80xc2x0 C. of not less than 25 MPa and a dynamic storage modulus (Exe2x80x2) at 120xc2x0 C. of not less than 10 MPa.
The substrate is not crosslinked during or after a forming process thereof.